The effect of ultraviolet light on articles such as retroreflective sheetings for use on license plates or signage articles that are exposed to the sun or other sources of ultraviolet light are of great concern to the manufacturers of such articles. Over time, constant or repeated exposure to ultraviolet light can result in fading of the dyes and pigments used in such articles and in the degradation or breakdown of the adhesives, polymers, and other materials used in the construction of the articles. This fading and degradation shortens the useful life of the articles in question, making protection from ultraviolet light exposure an issue of great importance to the manufacturers of such articles.
Molecules that function as ultraviolet light absorbers are generally known in the art. For example, in U.S. Pat. No. 4,853,471 Rody et al. describe 2-(2-hydroxyphenyl)-benzotriazole compounds that are useful as ultraviolet light absorbers. These compounds are described as useful in the stabilization of a wide variety of polymeric materials against ultraviolet light, particularly those that are used in the preparation of photographic materials. Other classes of ultraviolet light absorbers include the 2-hydroxybenzophenones and the diphenylcyanoacrylates.
While such compounds are known to absorb ultraviolet light, their incompatibility with the polyurethanes often used as the top film in multilayer sheetings has limited their long term usefulness in such constructions. The low solubility and/or high mobility of typical ultraviolet light absorbers in polyurethanes results in an inability to maintain high levels of these compounds in polyurethane films; the compounds tend to bloom or migrate out of the film, and the protective properties of the compounds are lost.
Therefore, a need exists for ultraviolet light absorbing materials that are compatible with and resist blooming out of polyurethanes, and for products such as polyurethane films and other articles that incorporate such a material.